1. Field of the Invention (Technical Field)
The present invention relates to the detection of oxygen in aircraft fuel inerting systems including the monitoring of the output of inerting gas from an air separation module of an on-board inert gas generation system, and the monitoring of oxygen content in the ullage (gas space between liquid fuel surface and top of fuel tank interior) of aircraft fuel tanks.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The effective prevention and control of fires and explosions originating in fuel systems during in-flight, maintenance, and post-crash are critical for both commercial and military aircraft. As liquid fuel is consumed, a space (also called ullage) containing air and fuel vapor is developed above the liquid fuel. This fuel-air mixture is potentially dangerous if a combustible or explosive composition coupled with favorable environmental factors and an ignition source are present. Methods to reduce the oxygen concentration in this fuel-air mixture are necessary for the survivability of both commercial and military aircraft. These methods typically displace the oxygen inside the ullage with nitrogen. Moreover, monitoring systems are also needed to accurately determine the oxygen content inside the ullage and the quality of the inerting gas (S.-J. Chen, et al., “Detection of explosive mixtures in the ullage of aircraft fuel tanks,” 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev., AIAA Paper No. 2004-0548, January 2004).
Replacing the fuel vapor-laden space (ullage) inside the fuel tank as the liquid fuel is being consumed with nitrogen-enriched air (NEA) minimizes fires and explosions due to potential ignition sources such as lightning strikes, artillery shells, static discharge, wiring sparks, and heating sources. Air separation technologies for the air separation module (ASM) include pressure swing adsorption, hollow fiber membrane, ceramic membrane, and cryogenic air separation. The NEA usually contains more than 90 percent nitrogen. The ullage oxygen concentration is recommended to be between 9-12 percent to eliminate potential in-tank fires and explosions due to all possible ignition sources (T. L., Reynolds, et al., “Onboard inert gas generation system/onboard oxygen gas generation system (OBIGGS/OBOGS) study, Part II: Gas separation technology—State of the art,” NASA CR-2001-210950, 2001).
An onboard inert gas generation system (OBIGGS) generally encompasses an ASM to generate NEA, a compressor, storage tanks, and a distribution system. The military has OBIGGS currently installed in the AH-64, C-5, C-17, F-22, and V-22 aircraft to reduce oxygen levels below the lower explosive limit inside the fuel tanks. From 1959 to 2001, seventeen commercial aircraft experienced fuel tank explosions that resulted in 542 fatalities worldwide (Aviation Rulemaking Advisory Committee, “Service history/fuel tank safety level assessment,” Task Group 1, 14 Jul. 1998; www.ntsb.gov/Pressrel/2001/010411.htm). OBIGGS is currently being considered for commercial aircraft to reduce the likelihood of in-tank fires and explosions. Ground and flight tests are already underway to design effective inerting systems for the commercial aircraft. Oxygen sensors for monitoring the NEA exiting the ASM of OBIGGS and inside the ullage are required to fully assess the effectiveness of inerting systems and the safety of fuel tanks. Oxygen sensors used in military aircraft are based on zirconium oxide and operate at a high temperature that is not suitable for commercial aircraft due to the potential ignition hazard from the sensor itself.
D. E. Cooper, et al., “Pressure, and temperature-compensating oxygen sensor,” U.S. Pat. No. 5,572,031 (1996), disclose an optically-based absorption spectroscopy with frequency modulation to measure oxygen concentrations in aircraft fuel tanks. The RF modulation frequency is in the MHz range, and demodulation frequency is at an integral multiple of the modulation frequency. The sensor system contained a reference cell for line-locking and calibration purposes, and a sample cell that is covered with a gas permeable membrane to prevent liquid fuel from entering the said cell. The pressure in the reference cell is derived from the measurement of temperature inside the cell. Temperature and pressure sensors are located in the sample cell.
K. Susko, “On-board fuel inerting system,” U.S. Pat. No. 6,634,598 (2003), discloses an nitrogen inerting system to reduce combustible and explosive mixtures that could be present in the ullage of aircraft fuel tanks. A fiberoptic probe based on chemical fluorescence actively monitors the partial pressure of oxygen in the ullage. The sensor probe does not introduce electrical current into the fuel tank. The head of the sensor probe is shielded from the liquid fuel using a gas permeable membrane or a baffle system. The whole sensor probe needs to be maintained within a narrowly defined band of temperature to assure accurate oxygen readings. This temperature is usually above the maximum temperature that the aircraft encounters during its operation either on the ground or in-flight.
The present invention improves on the art by using an optically-based oxygen detection system that comprises a widely-tunable diode laser, a photodiode detector, a measurement cell that is open to the environment, beam forming optics, liquid immersion prevention mechanisms, liquid removal mechanisms, and a digital signal processor-based electronics. The present invention does not require a reference cell for line-locking the laser wavelength or calibrating the sensor; wavelength modulation (kHz) instead of frequency modulation (MHz) is used; the measurement cell is open to the environment being measured to allow both liquid and gas to easily pass through, thus eliminating any diffusion time that is required of a gas permeable membrane; means for monitoring the integrity of the senor head and laser system are implemented; and means for providing a control feedback mechanism to the OBIGGS is incorporated. The invention is preferably used to monitor oxygen concentrations in the ullage of aircraft fuel tanks and/or at the outlet of an ASM.